Electrographic imaging and developing processes such as electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature. General descriptions are found in U.S. Pat. Nos. 2,221,776, issued Nov. 19, 1940; 2,277,013, issued Mar. 17, 1942; 2,297,691, issued Oct. 6, 1942; 2,357,809, issued Sept. 12, 1944; 2,551,582, issued May 8, 1951; 2,825,814, issued Mar. 4, 1958; 2,833,648, issued May 6, 1958; 3,220,324, issued Nov. 30, 1965; 3,220,831, issued Nov. 30, 1965; 3,220,833, issued Nov. 30, 1965; and many others. These processes involve the step of forming a latent electrostatic charge image on an insulating electrographic element. The electrostatic latent image is then rendered visible by treatment with an electrostatic developing composition or developer.
Electrostatic developers generally include a carrier that is triboelectrically chargeable. The carrier is a magnetic material such as iron filings, powdered iron or iron oxide on a non-magnetic substance like glass beads or crystals of inorganic salts such as sodium or potassium chloride. In addition to the carrier, developers include a toner which is electrostatically attractable to the carrier. The toner is generally a particulate polymeric material which is colored with a colorant such as carbon black for image viewing purposes.
To develop an electrostatic image, the dry developer is overall applied to the electrostatically charged surface by various techniques such as cascade development and magnetic brush development.
In conventional electrophotographic applications, the developed image is formed on a photoconductive element. If desired, the image is then transferred to a receiving sheet. The transferred image is then fixed, generally by heating to fuse the transferred image. Thus, the toner material must be capable of being fused under temperature conditions which will avoid any charring, burning or other physical damage to the receiver sheet which is generally paper.
The ability of the toner to fuse adequately at a relatively low temperature is quite important. This toner property is described herein as a low "onset of fusing" temperature. The use of a toner having this property allows operation at lower temperature, thus consuming less energy and increasing machine life by reducing the degradation effects of heat on elastomeric fusing roller materials, electronic components, and the like; and further allows higher-speed machine operation. Ideally, the toners should have an onset of fusing temperature which is just high enough to avoid coalescence at the temperature of machine operation.
"Off-setting" is the undesirable transfer of toner particles from the developed toner image carried on a receiving sheet to the surface of a heated fusing device. The surface of the fusing device becomes contaminated with toner particles. Upon further use of the contaminated fusing device, toner particles to the surface of the fusing member are transferred to subsequent receiving sheets. As a result, either a ghost image of previously fixed images is formed on subsequent receiving sheets, or undesirable deposits of toner material are formed in background areas of subsequent receiving sheets.
The temperature at which the cohesive strength of the toner matrix material (or binder resin) fails and the temperature at which the toner sticks to the fusing roller and causes off-set is referred to as the "hot offset" temperature. An elevated "hot offset temperature" is desirable for a useful toner. The difference between the "onset of fusing" temperature and the "hot offset" temperature is referred to herein as "offset latitude". The greater the offset latitude is, the wider the temperature range in which the fusing roller operates.
It is also desirable for the toner material to have good grindability. This means that the toner material should be sufficiently brittle to allow the toner material to be easily ground to the desired particle size.
Polyester resins have been broadly suggested for use as a matrix material or binder for electrographic toners. U.S. Pat. No. 4,140,644, issued Feb. 20, 1979 to Sandhu et al, discloses a polyester toner composition which possesses both a low onset of fusing temperature and a high hot offset temperature. In other words, this polyester composition possesses a wide "offset latitude". However, up to now, the art has not been able to provide a polyester toner composition which possesses both a wide offset latitude and good grindability.